It is known (see German Patent No. 3502248 and the corresponding U.S. Pat. No. 4,662,047 issued May 5, 1987) to provide a piston blank for a forged piston for an internal combustion engine with a piston head or a piston crown having a heat-throttling or heat-barrier annular gap defined along the inner side of the piston apron or collar which rides along the wall of the cylinder.
On the underside of the piston head or crown, a pair of connecting rod ears extend downwardly and can be considered to have eye portions remote from the piston head or crown and head portions proximal to the latter. In the eye portions, recesses can be formed in the casting and/or during the forging of the blank which can be machined to provide the bores which can accommodate the piston pin by means of which the connecting rod or a rocker arm or other piston rod may be pivotally connected to the piston between the ears.
The piston crown or head itself can comprise a piston cover lying inwardly of the collar or apron, and this collar or apron. On the underside of the cover, the blank can be formed with a rib or web bridging between the ears and constituting a so-called fire rib.
The heat-throttling annular gap is thus located between the piston crown collar and the piston shaft formed by the aforementioned ears and referred to hereinafter on occasion as the piston shaft ears.
In general, the forged piston blank is subjected to machining processes to form the finished forged piston, these machining processes including machining the outer surface of the collar or apron and the forming of the aforementioned bores in the regions constituted by the recesses described. It is desirable, obviously, to so form the piston blank that the machining operations can be held to a minimum.
In the prior art piston blank of this type, described and illustrated in the aforementioned patents, the piston shaft ears have a wall thickness which generally increases monotonically, i.e. continuously, from the lower end of the ear to its junction with the piston cover, i.e. the head of the piston.
While this does satisfy all of the requirements for enabling the ear to withstand the stresses to which it will be subjected both in the thermal and mechanical sense in a wide variety of engines, it does not satisfy the needs of more modern engines with larger cylinder bores and pistons of correspondingly larger diameters.
Indeed, modern developments in engine construction have required larger cylinder bores and pistons which must be subjected to greater stress and, as a consequence, larger diameter piston pins to connect the pistons to the connecting rods. Such larger diameter piston pins are necessary to keep the loading per unit area at a minimum and to be able to take up greater stresses in the connection between the rod and the piston.
As a consequence, the practice used in the past with respect to a continuous taper of the ears from the piston head, when applied to the larger diameter recesses and bores necessary to receive large diameter piston pins have not proved to be satisfactory and problems have been encountered in the ability of the piston to accommodate the high stresses both thermal and mechanical which arise in large diameter cylinders and other modern engines. The problem is all the more pronounced because it requires solving without significant modification of the outer geometry of the piston crown.